The invention relates to a method for reducing losses during the commutation of a free-running, driven power converter valve of an invertor phase to a current-accepting power converter valve of said invertor phase.
The publication entitled xe2x80x9cUse of the MOSFET Channel Reverse Conduction in an Invertor for Suppression of the Integral Diode Recovery Currentxe2x80x9d, printed in the Conference Report xe2x80x9cThe European Power Electronics Associationxe2x80x9d, 13. to 16.09.1993, in Brighton, pages 431 to 436 (xe2x80x9cEPExe2x80x9d), discloses a method by which losses are reduced during the commutation process. This known method is used in a polyphase invertor having Metal-Oxide Semiconductor Field-Effect Transistors (MOSFETs) as power converter valve.
MOSFETs are unipolar power semiconductors which are able to carry current in both directions. Every MOSFET has a parasitic bipolar freewheeling diode reverse-connected in parallel, said diode generally being designated as an inverse diode. This freewheeling diode has properties which are not optimal for the operation of the power converter valve, since it cannot be produced as a separate chip in a separate process. It is an integral part of the MOSFET. This inverse diode has a non-optimal on-state behavior and non-optimized stored charge.
FIG. 1 illustrates a known circuit of an invertor phase 2, which has a MOSFET in each case as power converter valves T1 and T2. The antiparallel bipolar freewheeling diode of the power converter valve T1 and T2 is designated by RD1 and RD2, respectively. On the DC voltage side, this invertor phase 2 is linked to a DC voltage source 4 across which a DC voltage UZK is dropped, and which is also designated as intermediate circuit voltage. The junction point 6 between the two power converter valves T1 and T2 that are electrically connected in series forms an AC connection to which a load can be connected. The MOSFETs used are normally off MOSFETs, which are designated as enhancement-mode MOSFETs. In n-channel enhancement-mode MOSFETs, a drain current flows only when the gate-source voltage UGS exceeds a predetermined positive value.
FIG. 2 illustrates a current/voltage characteristic of a MOSFET which is disclosed in the xe2x80x9cEPExe2x80x9d conference report. This current/voltage characteristic has different characteristic curves running in the quadrants I and III. That part of the characteristic curve in the quadrant I which is designated by Tc is used when the MOSFET is driven by means of a gate-source voltage UGS=15 V. That part of the characteristic curve in the quadrant III which is designated by TRCC is used when the MOSFET is driven and a load current ILOAD flows counter to the main direction through the MOSFET. If the MOSFET is not driven (UGS=0 V), then the characteristic curve in the quadrant III which is designated by TD is used. In other words, the integral freewheeling diode RD of the MOSFET carries the load current ILOAD.
In accordance with this characteristic, it can be seen that the on-state losses of a MOSFET can be reduced if the MOSFET is driven in free-running operation. As a result, the free-running current is divided between the transistor and the integral free-wheeling diode RD. This operation is characterized by the characteristic curve TRCCD in the quadrant III.
During the commutation process from the power converter valve T2, which is free-running and is driven, to the current-accepting power converter valve T1 (FIG. 1), it is necessary, in accordance with the publication xe2x80x9cCommutation Behaviour in DC/AC-Converters with Power MOSFETxe2x80x9d, printed in xe2x80x9cPCIxe2x80x9d, June 1986, pages 316 to 330, for the power converter valve T2 to be switched off before the power converter valve T1 is allowed to be switched on. This is necessary in order to prevent a short circuit as a result of the two power converter valves T1 and T2 being switched on simultaneously. This means that, at the instant of commutation, the integral freewheeling diode RD of the free-running power converter valve T2 carries the load current ILOAD and thus, on account of the stored charge, the freewheeling diode RD causes switch-off losses.
The xe2x80x9cEPExe2x80x9d publication specifies a method whereby the load current ILOAD during the commutation process from the free-running, driven power converter valve T2 to the current-accepting power converter valve T1 is not carried by the integral freewheeling diode RD2 of the power converter valve T2. This known method is characterized in that the current-accepting power converter valve T1 is driven so slowly that only a minimal current overshoot occurs. The slow driving of the current-accepting power converter valve T1 results in an increase in the switch-on losses of said valve. The level of these switch-on losses is dependent on the switch-on delay. The current overshoot is comparable to a diode reverse current which additionally loads the power converter valve T1. For this temporally extended driving, overcurrent detection is required for each power converter valve T1 and T2 of an invertor phase 2. This current in the bridge path is detected by means of voltage measurement on a leakage inductance. To that end, on the one hand the value of the leakage inductance must be known exactly, and on the other hand a fast integrator must be provided, at the output of which the value of the current in the bridge path is then present. Connected downstream of this integrator is a peak value detector which, on the output side, is connected to an overcurrent control device. This method reduces the amplitude of the reverse recovery current and the switching losses of the free-running, driven power converter valve during the commutation process.
The present invention is based on the object of modifying the known method in such a way that the above mentioned disadvantages no longer occur. This object is achieved according to the present invention by virtue of the fact that the current-accepting power converter valve is switched on at the beginning of the commutation process, and that the free-running, driven power converter valve is rapidly switched off as soon as the value of its drain voltage is equal to zero.
The drain voltage of the free-running power converter valve is required as measured value for this method. This measured value is used during the known desaturation monitoring, which detects a short-circuit current or an overcurrent. In other words, a further measured-value detection device is not required in order to be able to carry out the method according to the invention.
As a result of the driving of the current-accepting power converter valve, the load current commutates from the free-running power converter valve to the current-accepting power converter valve. The value of the drain voltage of the free-running power converter valve changes as a function of this current commutation. At the beginning of commutation, the drain voltage has a negative value of the order of magnitude of the saturation voltage of the power converter valve. At the end of the load current commutation, the entire intermediate circuit voltage is dropped as reverse voltage across this power converter valve, since the current-accepting power converter valve carries the load current. From these two cut-off values of the drain voltage, it can be seen that the profile of the drain voltage has a zero crossing during the commutation process. It is exactly at this instant that the load current completely commutates to the current-accepting power converter valve. In order that the switch-off losses are minimized as far as possible, the free-running power converter valve must be switched off as rapidly as possible at this instant. Depending on how rapidly this switch-off is effected, a parallel-path current flows through the free-running power converter valve and through the current-accepting power converter valve in addition to the load current. In other words, the losses that occur cannot be eliminated, but rather can only be reduced depending on how rapidly the free-running power converter valve is switched off. This reduction of the losses is significantly greater than the reduction by means of the known method, since, in the case of the known methods, on the one hand there are processing steps for the measurement signal and on the other hand the overcurrent control device can operate only when an overcurrent has already occurred.
In a preferred embodiment of the method of the present invention, the gate voltage of the free-running, driven power converter valve is decreased, at the beginning of the commutation process until its drain voltage is equal to a predetermined reference voltage. This additional method step improves the identification of the voltage zero crossing of the drain voltage since, irrespective of the value of the saturation voltage of the free-running, driven power converter valve, the initial value of the drain voltage at the beginning of the commutation process always has the value of the reference voltage. This becomes apparent particularly in the case of small load currents.